1. Field of the Invention
The present invention relates to an estimator of error rate which is suitably used for a radio receiving system for digital mobile communications or the like, and more particularly to the estimator of error rate which operates to estimate an error rate through the use of phase information and envelope information of a received baseband signal.
2. Description of the Related Art
A radio receiving system for digital mobile communications such as a portable phone is required to estimate an error rate, because the system has to perform various controls according to the error rate of the received signal. However, it is impossible or difficult to precisely estimate the error rate. Hence, the system employs a technique of estimating the error rate under the certain conditions. For example, if the received signal is close to a threshold value of a middle between signal points, under a certain condition, the received signal may be estimated as an error with a probability of 50 percents. Hence, by counting the signal closing to the threshold value, it is possible to estimate the error rate.
FIG. 9 shows an arrangement of a conventional estimator of error rate. In FIG. 9, a memory 1 operates to generate phase information 8 from a baseband signal 6 of an I channel (I signal) and a baseband signal 7 of a Q channel (Q signal). A delaying unit 2 operates to delay an input signal by one symbol period. A subtracter 3 operates to subtract the phase information of one symbol past from the phase information at the current time point. A coincidence detector 4 operates to generate an error pulse only if the input signal coincides with a signal for representing an error pulse generation area. A counter 5 operates to count the number of error pulses.
In the estimator of error rate arranged as described above, an I signal 6 and a Q signal 7 are inputted to the addresses of the memory 1. The I signal 6 is a baseband signal of the I channel derived by orthogonally detecting the received signal. The Q signal 7 is a baseband signal of the Q channel derived by the same manner. The memory 1 stores the phase information arctan(Q/I) of the I signal 6 and the Q signal. Then, by reading out the phase information from the memory 1, it is possible to obtain the phase information 8 of the I signal 6 and the Q signal 7.
Next, the phase information 8 is delayed by one symbol period through the effect of the delaying unit 2. This delay is required for demodulating an original signal from a differential phase modulated signal by the delaying detection. Then, the subtracter 3 operates to subtract the delayed phase information from the phase information 8 for deriving a delayed detection signal 10.
Next, the delayed detection signal 10 is inputted to the coincidence detector 4, in which if the signal 10 coincides with the signal 11 for representing the error pulse generation area, an error pulse signal 12 can be obtained. If the delayed detection signal 10 is close to a border value, the signal is estimated as an error signal with a probability of 50 percents. Hence, by setting the error pulse generation area to a value close to the border value, it is possible to obtain the information for estimating the error.
Lastly, an error pulse signal 12 is inputted into the counter 5. The counter 5 operates to count the number of error pulses at each burst signal and then supply a signal 13 for representing the number of error pulse counts.
The estimator of error rate arranged as described above uniformly sets the error pulse generation area relative to the phase information and generates the error pulse if the phase information coincides with the error pulse generation area. As a result, the estimator is requested to count signals that are less probably estimated as error ones if the other conditions are changed. Hence, the variations of the count value at each burst signal take place, which result in disadvantageously lowering an accuracy of estimating an error rate.
That is, the conventional estimator of error rate is arranged to estimate an error rate on the basis of only the fact that the phase information is close to the border value for determining the phase and to set certain values before and after the border value as the error pulse generation area. If the envelope information is close to a reference value, an error takes place only around the border value for phase determination. The probability that the received signal is not erroneous is high at both edges of the set error pulse generation area. Hence, the estimator counts the non-erroneous signals in some states of the envelope information, so that variations of the count value may take place.
On the other hand, the JP-A-59-200547 has disclosed provision of means for detecting an error of a channel by referring to a predetermined syndrome in a receive demodulating system apparatus located in a terminal apparatus and/or a relay apparatus. However, detection of an error on the syndrome needs a complicated calculation and a considerably long time and large circuit scale. Further, if the error rate is equal to or larger than a certain value, though an error itself can be detected, the number of errors, that is, the error rate cannot be grasped.